Motor vehicles are typically provided with servo-assistance of the braking system, so as to proportionately reduce the load applied to the brake pedal for a given braking effect. Many different kinds of servo-assistance have been proposed, but vacuum assistance via a vacuum brake booster is most common.
In such a booster, vacuum exhausts a brake booster chamber which is divided by a diaphragm. By opening one side of the diaphragm to atmosphere, a net force can be applied by the diaphragm to a piston of a hydraulic brake circuit. Suitable control means ensure assistance is proportional to the applied brake pedal load. The source of vacuum is typically a vacuum pump driven mechanically by the vehicle engine and/or engine inlet manifold depression.
More recently electric vacuum pumps have become preferred because they are independent of the vehicle engine and thus allow more flexibility in engine design and engine compartment utilization (packaging). Furthermore electric vacuum pumps can be switched on and off according to demand, and this can avoid the unnecessary energy consumption of an idling mechanically driven pump; switching on demand also gives a potentially longer service life for the pump.
A conventional electric vacuum pump for a vehicle brake booster incorporates a vacuum switch or sensor in the vacuum line to the brake booster, or in the brake booster chamber. A switch provides simple on/off control, whereas a sensor provides an electric signal which is an analogue of the depression available in the booster; both arrangements permit the pump to be switched on and off according to booster demand. The control system includes a degree of hysteresis to avoid continual pump activation/deactivation, and has fixed on and off values.
One switching strategy relies on percentages of absolute vacuum, which is typically −101 kPa (assuming atmospheric pressure at sea level to be 101 kPa).
On and off switching values are selected, for example at 65% and 80% of this maximum available vacuum (MAV), as indicated by the vacuum sensor or switch. In this arrangement on-switching occurs at −101 kPa*0.65=−65.6 kPa, and off-switching occurs at −101 kPa*0.80=−80.8 kPa. These fixed switching points are selected according to parameters of the vehicle braking system and the intended duty of the vehicle. The off-switching value is set to accommodate the achievable vacuum of a pump in good condition, which may be around 85% MAV.
At high altitude atmospheric pressure is reduced, with the possible consequence that, once switched-on, an electrical vacuum pump might not switch-off if the off-switching point is close to, or exceeds the maximum depression achievable by the pump. Continuous running of the pump may reduce pump efficiency, may have an effect upon pump life, and may result in high pump temperature. It would be advantageous to provide a solution to ameliorate or eliminate these high altitude effects.
It is known to compensate for altitude in vehicle systems which incorporate a vacuum pump.
US-A-2005/0178622 discloses a sensor of atmospheric pressure, and a look-up table which prescribes values of depression at which occur activation and deactivation of a vacuum pump. As altitude increases both the activation and deactivation values reduce. This arrangement provides step changes in the activation and deactivation values, and is somewhat complex in requiring the necessary control and memory apparatus, and initial calibration.
U.S. Pat. No. 4,412,416 discloses a diaphragm sensor having an internal coil spring operable to switch an electric vacuum pump on and off. This sensor provides altitude compensation, but has a fixed on/off range for all altitudes. As a consequence an on/off range which is suitable for sea level gives a somewhat wide on/off range at altitude, which means that the pump on value may be at a lesser depression than is desirable if the off value is within the capability of the vacuum pump. If however the range is reduced so that the on value at altitude is at a greater depression, the vacuum pump may run more frequently than is desirable, especially at lower altitudes.